This is the U.S. National Stage of International Application No. PCT/AT00/00072, which was filed on Mar. 24, 2000 in the German language.
The invention relates to a beamforming method for adaptive antenna arrays including several antenna elements in the downlink of frequency duplex systems, wherein antenna weights are determined for the antenna elements for downlink transmission on the basis of directional information of the uplink.
Furthermore, the invention relates to a beamforming device for adaptive antenna arrays including several antenna elements in the downlink of frequency duplex systems, comprising a signal processing unit used to determine antenna weights for the antenna elements for downlink transmission on the basis of directional information of the uplink.
It is known to electronically modify array antennas consisting of several individual antennas in respect to their directional characteristics in order to adaptively adapt the same to the respective channel situation in the optimum manner. Adaptive antennas initially were employed in radar technology, yet also their application in mobile communication systems has been investigated for quite some time. The use of adaptive antennas may lead to a reduction of the received interference by directed reception, a reduction of the generated interference by directed transmission and a reduction of the time dispersion of the mobile radio channel and hence a reduction of the intersymbol interference decisively codetermining the bit error rate.
These improvements may be used for a capacity gain, to increase the spectral efficiency, to reduce the necessary transmission power by the antenna array gain, to improve the transmission quality (reduced bit error rate), to increase the data rate and to extend the range of action.
Although not all advantages can be exploited at one and the same time, it is, nevertheless, feasible to achieve some of the above-mentioned improvements in each case. Thus, it would be absolutely essential to enable, by means of adaptive antennas, a more efficient utilization of the frequency spectrum available and, at the same time, an increase in the capacity and hence possible number of users in a cell at the same frequency band and the same number of base stations.
Mobile cellular wireless communication nets, in general, are limited in interference, i.e., the spatial reuse of one and the same radio channel, on the one hand, and the spectral efficiency, on the other hand, are limited by common channel interferers. A radio channel is defined by its frequency and/or its time slot (in the time multiplexxe2x80x94TDMAxe2x80x94time division multiple access) or its code (in the code multiplexxe2x80x94CDMAxe2x80x94code division multiple access). To supply more than one user by one and the same radio channel in TDMA and FDMA (frequency division multiple access) systems, methods based on the spatial divisibility and the direction-selective reception in the uplink (mobile station transmitting, base station receiving) as well as the direction-selective transmission of the user signals in the downlink (base station transmitting, mobile station receiving) have been proposed (socalled SDMAxe2x80x94space division multiple access system). The direction-selective transmission/reception in CDMA systems may also be used to increase the possible number of users on one frequency and hence raise the spectral efficiency and the capacity of a mobile cellular radio system. Thus, the possible number of users on a communication channel, that can be detected in the uplink by the base station through the linear adaptive antenna array and supplied in the downlink is increased with the interference remaining the same.
Three basic methods are known to divide the signals of the individual users by common channel interference suppression and detect the same: (1) Methods based on the knowledge of the spatial structure of the antenna array (socalled spatial reference methods), cf. R. Roy and R. Kailrath, xe2x80x9cESPRITxe2x80x94Estimation of Signal Parameters via Rotational Invariance Techniquesxe2x80x9d, IEEE Trans. Acoust., Speech and Signal Processing, Vol. 37, July 1989, pp. 984-995; (2) methods based on the knowledge of a known signal sequence (socalled temporal reference methods), cf. in S. Ratnavel, A. Paulraj and A. B. Constantinides, xe2x80x9cMMSE Space-Time Equalization for GSM Cellular Systemsxe2x80x9d, Proc. IEEE, Vehicular Technology Conference 1996, VTC 96, Atlanta, Ga., pp. 331-335; and (3) socalled xe2x80x9cblindxe2x80x9d methods using known structural signal properties for signal division and detection, cf. in A-J. van der Veen, S. Talwar, A. Paulraj xe2x80x9cA Subspace Approach to Blind Space-Time Signal Processing for Wireless Communication Systemsxe2x80x9d, IEEE Transactions on Signal Processing, Vol. 45, No. 1, January 1997, pp.173-190.
Various methods based on different estimates of the mobile radio channel are used for the downlink. In principle, either the directions of incidence of the signals of the mobile stationd (cf., e.g., U.S. Pat. No. 5,515,378 A or EP-755 090 A) are used, or the spatial covariance matrix (spatial correlation matrix) is used for beam formation (cf. U.S. Pat. No. 5,634,199 A).
A difficult problem is set by the different carrier frequencies in frequency duplex systems (FDD systems). In FDD systems, the signals both in the uplink and in the downlink are transmitted at different frequencies, thereby ensuring the necessary division between transmitted and received data both at the mobile and base stations. Due to the frequency difference, the antenna directivity pattern will be different, if the same physical antenna array and the same antenna weights (amplitude and phase) are used at different frequencies. For this reason, it is not advisable to use the same antenna weights for transmission and reception at the base station of a mobile cellular communication system. The exclusive use of the direction of incidence estimated in the uplink does not have any problems with that frequency offset, yet restricts beam formation to a single discrete direction of incidence, what is in contradiction to the physical nature of the mobile radio channel and, therefore, results in a limited capacity gain by the adaptive antenna. The use of the spatial covariance matrix of the uplink, however, involves the drawback of a frequency offset.
Various approaches have already been described to compensate for that frequency duplex distance in the spatial covariance matrix. Thus, it has been proposed to estimate in the uplink the direction of incidence, the signal power and the pertinent angular spread of each user, cf. T. Trump and B. Ottersten, xe2x80x9cMaximum Likelihood Estimation of Nominal Direction of Arrival and Angular Spread Using an Array of Sensorsxe2x80x9d, Signal Processing, Vol. 50, No. 1-2, April 1996, pp. 57-69. From that estimate for the uplink, an estimate of the spatial covariance matrix for the downlink is made, cf. also P. Zetterberg, xe2x80x9cMobile Cellular Communications with Base Station Antenna Arrays: Spectrum Efficiency, Algorithms and Propagation Modelsxe2x80x9d, thesis, Royal Institute of Technology, Stockholm, Sweden, 1997. That method, however, will function only if each mobile station has but a single nominal direction of incidence in respect to the base station. Due to reflections on mountains in rural areas or large building complexes in urban areas, this condition is frequently not met, thus rendering this approach inapplicable.
Another prior art proposal aims to use in the base station for transmission and reception in a frequency duplex system, two different antenna arrays scaled with the applied wavelength; cf. G. G. Rayleigh, S. N. Diggavi, V. K. Jones and A. Paulraj, xe2x80x9cA Blind Adaptive Transmit Antenna Algorithm for Wireless Communicationxe2x80x9d, Proceedings IEEE International Conference on Communications (ICC 95), IEEE 1995, pp. 1494-1499, or the corresponding WO 97/00543 A. There, the two xe2x80x9cadaptedxe2x80x9d antenna arrays, however, have to be manufactured and calibrated in a highly precise manner and placed in exactly the same position. Moreover, a second antenna array is required, thus raising costs superproportionally.
According to U.S. Pat. No. 5,634,199 A already mentioned above, the spatial covariance matrix of the downlink is to be measured directly by transmitting test signals from the base station and retransmitting the measured signals by the mobile station (cf. also W096/37975, which also refers to the transmission of test signals). However, that test signal method requires system capacity for the feedback process involved and, as a result, reduces any possible capacity increase. Furthermore, the standard of already existing mobile cellular communication systems would have to be changed, because no such feedback by the mobile cellular station has so far been provided in any mobile cellular communication system.
In U.S. Pat. No. 5,848,060 A the estimation of the spatial covariance matrix of the uplink from the reception signals of the same is described; the relative phases of the matrix elements occurring are then scaled by the ratio of transmission frequency to reception frequency (fS/fE). Due to the multipath propagation of the individual signals, the frequency, however, enters nonlinearly into the phase relation of the individual antenna elements. This application is, therefore, limited to cases where direct visual contact is provided between transmitter and receiver without reflections from different directions such as, for instance, in satellite communication.
In order to obtain a covariance matrix for the downlink, it was also proposed to apply a rotation matrix to the covariance matrix of the uplink, which rotation matrix corrects the phases of a wave coming from a defined direction by the ratio of transmission frequency to reception frequency fS/fE, cf. the already mentioned document G. G. Rayleigh, S. N. Diggavi, V. K. Jones and A. Paulraj, xe2x80x9cA Blind Adaptive Transmit Antenna Algorithm for Wireless Communicationxe2x80x9d, Proceedings IEEE International Conference on Communications (ICC 95), IEEE 1995, pp. 1494-1499. Yet, only the phase relation of a direction of incidence in respect to the base station is properly corrected there. If there are several different directions of incidence, that method will fail, wherefor it is applicable only in rural areas having a dominant direction of incidence.
The above-mentioned thesis by P. Zetterberg, xe2x80x9cMobile Cellular Communications with Base Station Antenna Arrays: Spectrum Efficiency, Algorithms and Propagation Modelsxe2x80x9d, thesis, Royal Institute of Technology, Stockholm, Sweden, 1997, also contains the proposal to apply a compensation matrix to the covariance matrix of the uplink. That compensation matrix is valid only for very small relative duplex distances 2(fSxe2x88x92fE)/fS+fE and is averaged over the whole region of the application angle of the adaptive antenna. That method does not correct the frequency difference, but only reduces the deviation, thus xe2x80x9cblurringxe2x80x9d the spatial structure of the mobile radio channel contained in the covariance matrix over the total angular region. Consequently, that method is not applicable at all.
Finally, it has already been proposed to decompose the covariance matrix of the uplink in Fourier coefficients and restore it at the transmission frequency, cf. J. M. Goldberg and J. R. Fonollosa, xe2x80x9cDownlink beamforming for spatially distributed sources in mobile cellular communicationsxe2x80x9d, Signal Processing Vol. 65, No. 2, March 1998, pp.181-199. That method tries to restore the exact phase relation of the individual signal paths at the transmission frequency, yet likewise blurs the spatial structure of the covariance matrix.
Thus, it is an object of the present invention to provide a method and a device of the initially defined kind, which efficiently enable such beamforming in the downlink of FDD systems so that the interferences also of the signals transmitted from the base station and received by the mobile stations may be reduced and the number of users to be supplied, i.e., mobile stations, may be increased.
To this end, the method according to the invention, of the initially defined kind is characterized in that the antenna weights for downlink transmission are determined on the basis of the power angle spectrum of the uplink of the individual users, wherein the power angle spectrum is modified by masking out undesired regions.
Correspondingly, the device according to the invention, of the initially defined kind is characterized in that the signal processing unit is arranged to determine the antenna weights for downlink transmission on the basis of the power angle spectrum of the uplink of the individual users upon modification of the former by masking out undesired regions.
In the technology according to the invention, downlink beamforming is, thus, based on the power angle spectrum of the uplink of the individual users with undesired angular regions being gated out in said power angle spectrum, i.e., possible interferers are blocked out in the power angle spectrum in order to ensure the optimum orientation of the main lobe in the direction of the respective user. Thus, according to the invention, the important, useful regions of the power angle spectrum are extracted and taken as a basis to determine the antenna weights for downlink beamformation. Investigations have revealed that particularly good results in regard to interference suppression will be obtained, if only one dominant part of the power angle spectrum is xe2x80x9ccut outxe2x80x9d of the same.
In doing so, it is advantageous if the power angle spectrum is estimated using a known signal sequence of the transmission signal, such as spread code, midamble, etc. It is also advantageous if the power angle spectrum of the uplink is estimated on the basis of the spatial covariance matrices of the uplink of the individual users or, optionally, their mean values. Furthermore, it has been shown to be beneficial if the respective spatial covariance matrix of the downlink is determined on the basis of the modified power angle spectrum of the individual users, or its mean value. Finally, it is advantageous if the spatial covariance matrix of the downlink, or its mean value, is used to calculate the antenna weights for transmission.
Thus, beamforming of the spatial properties of the mobile radio channel in respect to the spatial covariance matrix is preferably effected, which comprises the four steps of
estimating the spatial covariance matrix of the uplink;
determining the power angle spectrum by spectral search methods at the reception frequency;
reconstructing the spatial covariance matrix of the downlink using the estimated modified power angle spectrum at the transmission frequency; and
calculating the antenna weights for each user of the physical channel.
The technology of this invention is applicable in a manner unrestricted by the propagation conditions of the electromagnetic waves. It is not subject to any restrictions in respect to a single dominant direction of incidence for each user and may be implemented without any additional hardware equipment. There are no assumptions whatsoever as to the frequency difference between transmission and reception cases and, therefore, the technology described herein will function also independently of the relative duplex distance. In doing so, neither cumbersome iterative approximation procedures nor high-resolution direction estimation algorithms are required, thus providing a very calculation-effective solution.
In the following, the invention will be explained in more detail by way of examples and with reference to the drawing. Therein:
FIG. 1 is a schematic illustration of an adaptive antenna with downlink beam formation;
FIG. 2 schematically depicts a linear antenna array with an incident wave to illustrate path differences;
FIG. 3 schematically depicts a beamforming device, illustrating a base station and several mobile stations;
FIG. 4A shows an antenna pattern at an uplink frequency;
FIG. 4B shows the corresponding antenna pattern at the downlink frequency;
FIG. 5 is a flow chart illustrating the determination of the antenna weights for downlink beam formation;
FIG. 6 is a detailed flow chart elucidating the procedure during the frequency transformation represented in FIG. 5;
FIG. 7 shows the power angle spectrum of a user with xe2x80x9cinterferersxe2x80x9d;
FIG. 8 is an antenna pattern pertaining to FIG. 7 yet prior to modification;
FIGS. 9 and 10 are power angle spectrum and antenna characteristic diagrams corresponding to FIGS. 7 and 8, respectively, yet after masking out of an interferer; and
FIG. 11 schematically illustrates the structure of the signal processing unit used to calculate the antenna weights for beam formation.